In recent years, the optical communication technology has established its steady state as a high capacity, high speed signal transmission means by realizing an optical fiber as a medium for optical transmission with an ultra low propagation loss reaching the theoretical limit of 0.2 dB/km. The area of communication is not limited to be at-home and domestic but extends overseas, and is expanding rapidly as an information transmission means in a global network. Now it is a matter of time before it replaces the communication means by electric wires.
Development of low loss fiber material has played a crucial role in progress in the optical communication technology, but a further important progress is a progress in optical fiber amplifier. Although there are many functions of amplifiers, an optical amplification function is one of the most important technologies to fabricate optical networks. The optical amplification technology enables compensating the optical loss due to the optical fiber (Refer to Patent Documents 1 and 2 and non-Patent Documents 1 and 2 below). By placing fibers having the optical amplification function at relay stations arranged at an appropriate distance inbetween, information signal can be transmitted with no distance limitation in principle, at a speed of light and without need to convert optical signal to electrical signal. As a result, optical devices with high functionality and various optical systems can be realized. At present, the network of the optical fiber communication system reaches as long as not less than 10,000 km, and a feasible information amount per unit time is as high as 40 Gbit/s. By a progress in wavelength multiplexing technology, a transmission system with multiplexing of 256 wavelengths multiplexing is made possible, and consequently the transmission capacity of data through a single optical fiber extends to exceed 10 Tbit/s.
Furthermore, an optical fiber amplifier can be operated as a laser. The optical fiber laser has already expanded to many aspects such as a single wavelength laser, a multiple wavelength laser, a short pulse laser, and a high power laser. From a laser doped with rare earth element, as much output power as needed is available in principle if it is excited strongly. Such a high output power laser is useful as an excitation light source for other optical amplifier and an optical nonlinear device, and receives much attention as an alternative of the high power solid state laser now under use in industry. An Yb-doped optical fiber laser with continuous wave output power exceeding 1 kW has already been reported (See non-Patent document 3 below.).
[Patent Document 1] Japanese Patent Application Publication No. 2001-77451
[Patent Document 2] Japanese Patent Application Publication No. 2003-523457
[Non-Patent Document 1] M. Nakazawa et al., Appl. Phys. Lett., vol. 54, 295 (1987)
[Non-Patent Document 2] “Erbium Doped Optical Fiber Amplifier” Ed. by Sudo, Optronics Co. (1999)
[Non-Patent Document 3] Y. Jeong et al., Electron. Lett., vol. 40, 470 (2004)
[Non-Patent Document 4] K. Onodera et al., Tokin Tech. Rev., vol. 21, 33-36 (1994)